This application is directed to transmission line circuit structures, such as may be used for combiner/divider circuits, baluns, and the like, and particularly to planar circuit structures. High power broadband communication systems require high power broadband antennas. Often these antennas have an input impedance that does not match the desired transmitter or receiver with which it is used. In such circumstances, baluns can be used to transform the impedance of the antenna to the impedance of the transmitter or receiver, or to convert between an unbalanced signal and a balanced signal. When large bandwidths are desired, coaxial baluns are often used.
Simple signal sources have two terminals, a source terminal and a return terminal, where most commonly a ground plane is used for the return path. The ground plane return simplifies circuit wiring, as a single conductor and the ground plane form a complete signal path. The voltage on the ground plane is then the reference for this signal. Often this is referred to as an “unbalanced circuit,” or “single ended circuit.” In such unbalanced circuits, when wires cross or run parallel with one another, there can be undesired coupling.
One method for reducing such coupling is to use two conductors, one for the signal, the other for the signal return path, eliminating the ground plane return path. This is referred to as a “balanced” or “differential” circuit. In AC signals, either conductor can be considered to be the signal, and the other the signal-return. To minimize coupling to other circuits, it is highly desired that the signal current flowing in the two conductors be exactly the same, and 180 degrees out of phase. That is, all of the return current for one conductor of the pair is carried by the other conductor, and the circuit is balanced. This results in zero current being carried by the ground plane. In practice, such perfectly balanced currents are only a theoretical goal.
An amplifier that uses balanced or differential input and output connections is less likely to have oscillations caused by coupling of the input and output signals, and will have less extraneous noise introduced by the surrounding circuitry. For this reason, practically all high gain operational amplifiers are differential. A “balun” (short for “balanced-unbalanced”) is a component that converts between an unbalanced source and a balanced one. Some baluns are constructed with nearly complete isolation between the balanced terminals and ground. Some baluns are constructed with each balanced terminal referenced to ground, but with equal and opposite voltages appearing at these terminals. These are both valid baluns, but in the first case, the unbalanced voltage encounters high impedance to ground, making unbalanced current flow difficult, while in the second, any unbalanced current encounters a short circuit to ground, minimizing the voltage that enters the balanced circuit.
Microwave baluns can be either of these types, or even a mixture of the two. In any case, one could connect two equal unbalanced loads to the two balanced terminals, with their ground terminals connected together to ground. Ideally, the unbalanced signal input to the balun would be equally distributed to the two unbalanced loads. Thus, a balun may accordingly be used as a power divider or combiner, such as in a hybrid circuit, where the two unbalanced loads or sources connected to the balanced terminals are operating 180 degrees out of phase.